1. Field of the Invention
The present invention relates to a fuel tank. More specifically, the invention relates to a valve device, such as a backflow prevention valve (or check valve), to be attached to a resin fuel tank.
2. Related Art
Various valve devices, such as a backflow prevention valve, and a valve device for an evaporator circuit, are attached to an automotive fuel tank. In recent years, the resinification of fuel tanks has progressed. Thus, generally, these valve devices are joined to the resin fuel tank by welding.
To prevent fuel contained in a fuel tank from backflowing into a fuel feed pipe, a backflow prevention valve is interposed between the fuel tank and the fuel feed pipe. As illustrated in, for example, FIGS. 7 and 8, the backflow prevention valve consists of a cover 100, a case 200, a valve 300, and a spring 400.
The cover 100 has an outer side portion 101 made of maleic acid modified polyethylene, and an inner side portion 102 made of polyamide. The cover 100 is formed by dichromatic molding. The cover 100 has a flange portion 103 and a cylindrical portion 104 protruding from a surface of the flange portion 103. A ring-like first welding portion 105 to be welded to the fuel tank is formed in a circumferential edge part of the flange portion 103. A ring-like second welding portion 106 to be welded to the case 200 is formed in the inner side portion 102 on the inner circumferential side of the first welding portion 105.
The case 200 is formed of polyamide which is hardly swelled by fuel. Three support plates 201 are radially protruded in the case 200 at circumferentially equal intervals. A cylindrical boss 202 including a through hole is connected to a leading end of each of the three support plates 201. A ring-like welding groove 203 to be welded to the second welding portion 106 is formed in an end part of the case 200. A ring-like seal projection 204 is formed in the other end part of the case 200.
A valve 300 is constituted by a valve body 301 formed of polyacetal, a ring-like rubber seal seat plate 302 made of polyacetal, a perforated disk-like holding plate 303, and a cap 304. A valve body 301 has a flange portion 305, a shaft portion 306 protruding from the center of the flange portion 305, and an inner cylindrical portion 307 protruding from the flange portion 305 in a direction opposite to the shaft portion 306. Three claw portions 308 are formed on the inner cylindrical portion 307 at circumferentially equal intervals. Each of the claw portions 308 can be radially elastically deformable due to slits (not shown) in both sides thereof in the inner cylindrical portion 307.
The valve body 301 is such that the inner cylindrical portion 307 is inserted through both a central hole of each of the seal seat 302 and the holding plate 303. Each of the claw portions 308 engages with the holding plate 303, so that the seal seat plate 302 is sandwiched between the flange portion 305 and the holding plate 303. The outside diameter of each of the seal seat plate 302 and the holding plate 303 is larger than that of the flange portion 305. A part of the seal seat plate 302 protrudes from the outer circumference of the flange portion 305.
This valve 300 is such that the shaft portion 306 is inserted into the through hole of the boss 202 from below, as viewed in FIG. 8, so that a spring 400 is passed through onto the shaft portion 306 passing through the through hole. Additionally, the cap 304 is fixed to and is fit onto the head part of the shaft portion 306. Consequently, the spring 400 is held in a state in which the spring 400 is compressed between the cap 304 and each of the three support plates 201. Also, the second welding portion 106 of the cover 100 is welded to the welding groove 203 of the case 200. The valve device is configured in this manner. Additionally, the valve device is assembled to the fuel tank by inserting the case 200 into the fuel tank from an opening portion formed therein and by welding the first welding portion 105 to the outer surface of the fuel tank.
In a state in which no external force acts upon this valve device, the valve 300 is pushed by the spring 400 in a direction in which the valve 300 approaches the cover 100. Thus, the part of the seal seat plate 302, which is protruded from the outer circumference of the flange portion 305, is pressure-contacted with the seal projection 204, so that the valve is sealed.
A valve described in, for example, JP-A-2004-028043 is known as a backflow prevention valve of such a kind.
However, it is necessary for the above-described conventional valve device that the seal seat plate is formed of an elastic material, such as rubber or thermoplastic elastomer. The conventional valve device has drawbacks in that the number of components of the valve itself is large, and that the number of steps of a process of assembling the valve device is large.
A valve device, in which a seal surface is formed integrally with a resin valve element itself, is described in, for example, JP-A-2006-123576. This valve device is configured to form a surface of a valve seat formed in a case and a seal surface of the valve element into spherical surfaces having substantially the same center of curvature, respectively, and to pressure-contact the seal surface of the valve element with the valve seat so that the touch area between the seal surface and the valve seat is large.
However, this valve device, which is configured so that both the case and the valve are formed of resins and are of a spherical seal surface structure having the same center of curvature, as described in JP-A-2006-123576, has a problem that even when fuel swelling is small, the position of the seal surface is displaced from a seal contact position at which the valve is sealed, so that the valve is not sealed.